1. Field of the Invention
This invention relates to a method for making interconnect structures, particularly in a semiconductor integrated circuit.
2. Background Art
A Method for making interconnect structures comprises
forming a conductive layer;
forming of an insulating layer above said conductive layer;
creating a plurality of holes in said insulating layer and filling the holes with tungsten thereby forming tungsten plugs, such that said tungsten plugs are in electrical contact with said conductive layer;
forming a patterned metallisation layer that overlies said insulating layer and said tungsten plugs by means of following steps:
forming a continuous metallisation layer,
forming an organic mask,
etching in plasma said continuous metallisation layer,
removing the organic mask in a dry way, and
immersing the obtained wafer including the layers and the tungsten plugs in a cleaning solution to remove any post-etching residues.
Methods of this kind are generally used for fabricating reliable interconnect structures in semiconductor integrated circuits.
The insulating layer, mostly silicon dioxide, is conventionally deposited over the conductive structures.
Within this insulating layer holes are created and subsequently filled with tungsten such as to contact this first conductive layer.
The metallisation layer overlying the insulating layer and the tungsten plugs is usually patterned using the conventional photolithography and plasma etching techniques.
Photolithography is typically accomplished by depositing a photoresist layer, selectively exposing the photoresist layer to light through a patterned reticle, developing the photoresist layer, including wet removal of the non exposed photoresist and the developing products, to form a photoresist mask. This is followed by plasma etching of the exposed metal, removal of the photoresist mask and by removing in a cleaning solution of residues produced during the plasma etching step.
It may happen that the metallisation layer as well as the conductive layer are not electrically connected to the substrate. Consequently, these layers and the tungsten plugs will be electrically charged during the plasma etching. During the immersion of the wafer in the cleaning solution, exposure of the tungsten plugs is possible, for instance due to misalignment of the photoresist mask. If any of the tungsten plugs is exposed to this cleaning solution, the tungsten material might erode away.
Methods have been developed in the prior art wherein the erosion of the tungsten plugs is avoided.
U.S. Pat. No. 6,153,531 discloses a method for making interconnects of the above mentioned kind, wherein erosion of the tungsten plugs is avoided by submersing the whole wafer in a dilute nitric acid solution before immersing it in the cleaning solvent. Said immersion in a dilute nitric acid takes place until a tungsten oxide passivating film is formed above the tungsten that is not covered by the metallisation layer. This film is insoluble in the cleaning solvent.
Nitric acid is however a highly corrosive, environment-unfriendly material. Another disadvantage of this method is that, apart from the expenses on renewal of the acid, a high consumption of de-ionised water is required for rinsing the wafer after this acid treatment. The passivation also requires a very precise control of the pH of the acid, which is an expensive process to support in production.
Another method for making interconnects while avoiding the erosion of the tungsten plugs is disclosed in U.S. Pat. No. 6,077,762. Before submerging the semiconductor substrate in a cleaning solvent to remove the post plasma etching polymer residues, the electric charge of the metallisation layer built-up during the plasma etching is neutralised by contacting this layer with a conductive liquid, particularly a liquid metal such as mercury, that is electrically grounded. However, the use of a liquid conductive metal is rather non conventional, complicated and expensive.
U.S. Pat. No. 6,093,658 discloses still another method for making interconnects wherein, before submersion into the cleaning solvent, the obtained wafer is exposed to an electron dose of 5.000 xcexcC/cm2 to 25.000 xcexcC/cm2 that is configured to neutralise the electric charge built-up in the metallisation layer. This method however requires an electron beam system which is no typical system used in the manufacturing of semiconductor IC""s and is relatively expensive.
The invention seeks to provide a simple and economic method for making interconnect structures with tungsten plugs which do not erode.
In accordance with the invention, this object is accomplished by submitting, before submerging into the cleaning solvent, the wafer to a plasma containing F, H or a mixture of both.
Plasma equipment are conventional tools in the manufacturing of Integrated Circuits and each manufacturer of IC""s has one or more of these tools. Plasma treatment is well known in the technical field using field proven equipment and is simple, economic and environment-friendly and more flexible for automatisation approach compared to the above mentioned prior art solution.
In another variant, this plasma treatment is combined with said dry removal of the organic mask in a plasma containing F, H or a mixture of F and H.
This combined treatment permits to simplify further the manufacturing of the interconnect structure and the costs can be further reduced. Such combination would not be possible if other techniques than a plasma treatment are used to avoid erosion of the tungsten plugs.
When the plasma contains fluorine, the recommended source of Fluorine in the plasma is CF4, NF3, SF6, CHF3, or C2F6 gas.
When the plasma contains Hydrogen, the recommended source of Hydrogen in the plasma is Forming gas (a mixture of H2 and N2) or water vapour (H2O).
Oxygen, Oxygen containing gas or some neutral gas, like Argon, Helium and Nitrogen may be added to the plasma.